make readonly/disable tinymce textarea

Use the configuration parameter readonly

tinyMCE.init({
        ...
        theme : "advanced",
        readonly : 1
});

Here is a link to a demo.

Update: What you can do to prevent users from editing content in your editor is to set the contenteditable attribute of the editors iframe body to false:

tinymce.activeEditor.getBody().setAttribute('contenteditable', false);

From version 4.3.x on you can use code below for readonly mode

tinymce.activeEditor.setMode('readonly');

and for design mode:

tinymce.activeEditor.setMode('design'); 

IF you only have one editor, this works:

tinymce.activeEditor.getBody().setAttribute('contenteditable', false);

If you have multiple editors, you have to select them by the id of the textarea:

tinyMCE.get('textarea_id').getBody().setAttribute('contenteditable', false);